The present invention relates to noise attenuation panels and to a method of manufacturing a noise attenuation panel.
Noise attenuation panels are widely used for attenuating noise produced by aircraft engines and are located at optimised positions in the flow ducts of aircraft engine nacelle structures. Such flow ducts primarily comprise the inlet duct, the fan duct and the nozzle assembly.
A typical noise attenuation panel comprises a sound reflecting solid backing plate or sheet, a perforate metal facing plate or sheet and a honeycomb or cellular core which is bonded between the backing and facing sheets which partitions the air into a multiplicity of separate cells.
When the noise attenuation panel is mounted in a flow duct of an aircraft engine nacelle structure with the facing sheet exposed to sound waves generated in the duct, the sound waves become subjected to three mechanisms which result in a reduction of the sound energy by conversion thereof to heat energy, namely:
(i) friction in the facing sheet,
(ii) pressure loss when duct pressure sound waves expand into the cells of the honeycomb or cellular core, and
(iii) xe2x80x98reactivexe2x80x99 cancellation of the direct incident sound wave by the wave that is reflected from the solid backing sheet, the honeycomb cell depth being xe2x80x98tunedxe2x80x99 to the required frequency.
Noise attenuation panels are obviously important acoustically but, because of the hostile environment in which they operate, there is also an evident need for structural rigidity. As they form part of an aircraft engine nacelle structure it is important that the complete component is provided with adequate strength to withstand the inflight conditions to which an exposed area of a nacelle structure is susceptible. Moreover, noise attenuation panels are often so configured as to add strength to the nacelle structure into which they are to be installed.
The perforate facing sheets of noise attenuation panels heretofore proposed have commonly been perforated by punching or mechanical drilling. Current noise attenuation panel constructions use perforate facing sheets with holes typically of diameter between 0.020xe2x80x3 and 0.060xe2x80x3 positioned in an equi-spaced triangular array such as to provide open areas within the limits of 3 and 20%.
The porosity of the facing sheet of a noise attenuation panel may be designed to meet specific flow resistances by either changing the hole size, the spacing between the holes or by changing both. Acoustic research has demonstrated that hole size and open area are critical with regard to the attenuation properties of noise attenuation panels.
Current manufacturing procedures do not realistically allow the provision of very small hole diameters and very small spacing. In metal sheets, punching for example dictates a minimum hole diameter of 0.020xe2x80x3. Mechanical drilling can produce holes with diameters as low as 0.010xe2x80x3, but this is highly impractical with the lowest practical hole diameter being 0.020xe2x80x3.
It is an object of the present invention to provide a method of manufacturing a noise attenuation panel in which the holes in the facing sheet can readily be made with diameters and spacings smaller than that which can be achieved by punching or mechanical drilling.
The holes provided in perforate facing sheets of heretofore proposed noise attenuation panels are susceptible to blockage by particles. Such blockages of the gaseous fluid communication flow paths between the cells of the cellular core and the front face of the facing sheet reduces the effectiveness of the panel to attenuate noise. Where holes in the facing sheet are formed with very small diameters, they become even more susceptible to blockage which then places a limit on the smallest hole diameter which can be successfully used in practical applications.
It is another object of the present invention to provide a noise attenuation panel and a method of manufacturing a noise attenuation panel in which the holes in the facing sheet of the panel are formed with diameters and spacings smaller than those which can be achieved by punching or mechanical drilling and which are so configured as to be less susceptible to blockage than holes of the same diameter and not so configured.
According to a first aspect of the present invention there is provided a method of manufacturing a noise attenuation panel which comprises:
a cellular component part having a front face, a rear face and a structure which defines a multiplicity of cells between the front face and the rear face, and
a facing component part which:
has a front face and a rear face,
extends across the ends of the cells of the cellular component part at the front face thereof with the rear face of the facing component part adjacent the front face of the cellular component part, and
is provided with a multiplicity of holes therethrough to provide gaseous fluid communication between the cells of the cellular component part and the front face of the facing component part for the attenuation of noise generated by gaseous fluid flow over the surface of the front face of the facing component part,
the method comprising:
a first processing step of forming the cellular component part,
a second processing step of forming the facing component part, and
a third processing step of bringing the two component parts together in the formation of the noise attenuation panel,
characterised in that:
the second processing step comprises subjecting a blank facing component part to electron beam drilling at an entrance face thereof to produce a multiplicity of drilled holes through the facing component part from the entrance face to an opposite exit face under conditions in which each drilled hole has a bore diameter which decreases from the entrance face to the exit face throughout the bore or at least a part of the bore and which does not increase at any point of the bore, and
the third processing step comprises bringing the facing component part formed in the second processing step and the cellular component part formed in the first processing step together in such disposition that the exit face forms the front face of the facing component part and the entrance face forms the rear face of the facing component part.
In an embodiment of the invention according to its first aspect hereinafter to be described, the second processing step comprises subjecting the facing component part to electron beam drilling to produce a multiplicity of drilled holes through the facing component part in which each drilled hole has a bore diameter which decreases in the region of the exit face of the facing component part to a minimum bore diameter at the exit face.
In a preferred embodiment of the invention according to its first aspect the second processing step comprises subjecting the facing component part to electron beam drilling to produce the multiplicity of drilled holes with bore diameters not greater than 0.020xe2x80x3. The electron beam drilling may furthermore advantageously produce the multiplicity of drilled holes with bore diameters in the range of 0.002xe2x80x3 to 0.020xe2x80x3.
In an embodiment of the invention according to its first aspect, the second processing step comprises subjecting the facing component part to electron beam drilling to produce the multiplicity of drilled holes having a spacing between adjacent holes not greater than 0.11xe2x80x3. The electron beam drilling may advantageously produce the multiplicity of drilled holes having a spacing between adjacent holes in the range of 0.004xe2x80x3 to 0.11xe2x80x3.
According to a second aspect of the invention, there is provided a noise attenuation panel produced by the method according to the first aspect of the invention.
According to a third aspect of the invention there is provided a noise attenuation panel which comprises:
a cellular component part having a front face, a rear face and a structure which defines a multiplicity of cells between the front face and the rear face,
a facing component part which:
has a front face and a rear face,
extends across the ends of the cells of the cellular component part at the front face thereof with the rear face of the facing component part adjacent the front face of the cellular component part, and
is provided with a multiplicity of holes which extend through the facing component part from the front face to the rear face to provide gaseous fluid communication between the cells of the cellular component part and the front face of the facing component part for the attenuation of noise generated by gaseous fluid flow over the surface of the front face of the facing component part,
characterised in that:
each of the multiplicity of holes in the facing component part has a bore diameter which increases along the bore in the direction of the rear face of the facing component part throughout the bore or at least a part or parts of the bore and which does not at any point of the bore decrease along the bore in the direction of the rear face of the facing component part.
In an embodiment of the invention according to its third aspect, each of the multiplicity of holes in the facing component part has a bore diameter which decreases in the region of the exit face of the facing component part to a minimum bore diameter at the exit face.
In an embodiment of the invention according to its third aspect, the facing component part is formed with the multiplicity of holes prior to the bringing together of the facing component part and the cellular component part to form the panel and the holes are formed by electron beam drilling from an entrance face of the facing component part through to an exit face and the facing component part and the cellular component part are assembled to form the panel in such disposition that the exit face forms the front face of the facing component part and the entrance face forms the rear face of the facing component part.
In an embodiment of the invention according to its third aspect, the holes in the facing component part have bore diameters not greater than 0.020xe2x80x3. The holes in the facing component part may advantageously have bore diameters in the range of 0.002xe2x80x3 to 0.020xe2x80x3.
In an embodiment of the invention according to a third aspect, the spacing between adjacent holes in the facing component part is not greater than 0.11xe2x80x3. The spacing between adjacent holes may advantageously be in the range of 0.004xe2x80x3 to 0.11xe2x80x3.
In an embodiment of the invention according to its second and third aspects, the cellular component part has a wall structure which defines a multiplicity of juxtaposed cells which extend from the front face to the rear face of the cellular component part and the panel further comprises a backing component part extending across the rear face of the cellular component part.
According to a fourth aspect of the present invention, there is provided an aero engine having an engine core and a nacelle structure surrounding the core and defining a fan duct having inner and outer fan duct walls, one or each of the fan duct walls being formed by or including a noise attenuation panel according to the third aspect of the invention.